Stop a turn locking out its own memory bank, and let the long run notice

The first M01 trial with the memory bank on was 26 turns on a GPU host. It
accepted every turn and reported "complete". It also wrote two memories and
no summary, and logged 180 `database is locked` errors, while derived status
still read `idle`.

The cause was a single uncommitted UPDATE. Retrieval bumped each used
memory's counter before the model call, and the turn commits only after the
reply has streamed. SQLite has one writer, so the turn held the write lock for
the whole reply. Every post-turn memory, summary and status write in that
window waited out the five-second timeout and failed. Recording the failure
needed a write as well, and without a rollback first it raised
PendingRollbackError. The loss therefore reached the log and never reached
the status the Insights panel reads, which F08 forbids. The draco run never
hit this because the bank was off there.

- `retrieve_memories` now only reads. `record_use` writes the counters in the
  turn's single commit, so a turn that never lands counts nothing.
- The post-turn task's outer handler rolls back before it records a failure.

The harness could not have caught any of this. It read three prompt sections
under names the builder does not use: `memories` (really `used_memories`),
`story_history` (really `history`/`recent_history`), and a `knowledge` prefix
that matched the fixed instruction section instead of the imported passages.
Memory tokens read 0 whatever the prompt held, and the in-history and
in-memories recall checks could never come out true. The labels are now
constants, pinned by a test against a prompt the real builder assembled.

The harness also stops at the first sign of failed post-turn work. It checks
/derived and new server.log lines after every turn, keeps its log position
across --resume, and waits for background work to settle before its final
checks. A run with no memories or no summaries now ends "failed", not
"complete".

Both new application tests fail on fec46f6: the lock probe sees
`database is locked`, and memory status stays `idle`. The full backend suite
passes (1392 passed, 17 skipped). A 26-turn re-run against the same host had
0 lock errors, wrote 7 memories and 2 summaries, and used them in the prompt
from turn 8.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0136VBTMUKWYeU6G9HgbDbND

backend/tests/test_embedding_model_switch.py

@@ -138,7 +138,7 @@ def test_retrieval_uses_no_stale_vector_after_the_switch(client, monkeypatch):
         adventure = db.get(models.Adventure, client.adv_id)
         settings = db.query(models.Settings).first()
         result = asyncio.run(
-            memorybank.retrieve_memories(adventure, settings, update_stats=False)
+            memorybank.retrieve_memories(adventure, settings)
         )
         assert result["used"] == []
     finally: